Oxidizer-fuel combustor injectors such as the type typically used on hydrogen fueled rockets typically operate at low oxidizer-to-fuel ratios. During use, these injectors reach extremely high temperatures which are detrimental to the injector. Therefore, the firing time for these injectors must be limited unless the injectors are somehow cooled.
The most common method of cooling the injectors is to provide a porous metal face place which is transpiration cooled by fuel seeping through the interstices of the face plate. This works well for fuel rich injectors, since the transpiring fuel does not combust at the surface of the face plate but instead migrates into the combustion chamber before igniting.
Developments in combustion systems have lead to combustor injectors which operate at high oxidizer-to-fuel ratios. For these injectors, the normal density porous metal face plate leaks too much flow. If the porous metal is packed denser it is difficult to maintain an even porosity, resulting in uneven distribution of the transpiration cooling flow. Additionally, when the injector elements are brazed into the porous metal, the braze material wicks into the porous metal and blocks its passages.
What is needed is a cooled face plate for high oxidizer-fuel ratio gas injectors.